The present invention relates to a bonding film member of a film structure for bonding an electronic component onto a circuit board by supplying bonding material such as cream solder or the like onto land-shaped conductive layers of the circuit board, and a method for mounting electronic components onto the circuit board with the use of the bonding film member.
In order to mount a Quad Flat Package (QFP), e.g., an IC or a chip onto a circuit board, generally, cream solder is preliminarily printed on land-shaped conductive layers of the circuit board in a printing process, and leads of the electronic component are overlayed onto the cream solder in a setting process, and then the circuit board is heated to melt the cream solder in a reflow process.
The aforementioned processes will be explained more in detail with reference to the corresponding drawings.
In the printing process, as shown in FIG. 15, a metallic mask 2 is set on a circuit board 1, onto which cream solder 3 is supplied. When a squeegee 5 is moved in a direction of an arrow 6, the cream solder 3 is filled in the form of a film into openings 4 of the mask 2 formed in a predetermined pattern. The printing process is completed when the mask 2 is removed. In consequence, a layer of the cream solder 3 is formed on land-shaped conductive layers 7 of the circuit board 1 as indicated in FIG. 16.
Referring to FIG. 17, in the setting process, leads 100 and 10 of electronic components 90 and 9 which are transferred onto the circuit board 1 by means of a suction nozzle 8 are placed on the cream solder 3. Because of the adhesive force of the cream solder 3, the electronic components 90 and 9 are combined with the circuit board 1 via the cream solder 3, and then sent to the reflow process.
In the reflow process shown in FIG. 18, the circuit board 1 is heated by a heat source such as a hot wind or an infrared heater, etc. As a result of this heating, the cream solder 3 is melted, so that the leads 100 and 10 of the electronic components 90 and 9 are soldered and bonded to the conductive layers 7 of the circuit board 1, as in FIG. 19.
In accordance with the recent tendency toward more compact and higher-density electronic components, the pitch of the leads is increasingly narrowed and consequently, the size and the pitch of openings formed in the mask used in the above printing process are also reduced. As a result, when the mask is removed, it often takes place that the cream solder is partly left in the openings of the mask without being completely printed on the circuit board as shown by reference numeral 31 in FIG. 16, or part of the cream solder penetrates under the mask as shown by reference numeral 32 in FIG. 16. If such an undesirable accident is brought about in printing of the cream solder in a super fine pattern, blurs and bridges of the cream solder may be generated as shown by 33 and bonding becomes poor in the reflow process, causing shortcircuits and the like.
Meanwhile, the suction nozzle carrying the electronic components onto the circuit board may become inclined when pressing the electronic components against the circuit board, and therefore the cream solder may become deformed and spread in the widthwise direction of the conductive layers.
When the circuit board is heated in the reflow process, the flux of the cream solder is softened and comes out beyond the conductive layers. The shape of the cream solder is readily deformed, that is, sagging can easily occur.
As mentioned hereinabove, the defective bonding due to the insufficiency or deformation of the cream solder is induced not only in the printing process, but in the setting process and reflow process. Therefore, it is not easy to highly accurately mount the electronic components on the circuit board, particularly when the leads of the electronic components are arranged with a narrow pitch.
Some QFPs nowadays are arranged with pitches of their leads as small as 0.3 mm. A metallic mask of about 0.15 mm thickness is used in the printing process to mount QFPs with a 0.5 mm pitch of the leads or chip parts of 1.0 mm.times.0.5 mm size on circuit boards. Since the printing thickness is determined by the thickness of the mask, the printing amount, namely, the amount of the solder is regulated only by the size of the openings. However, the solder is required to be so little in mounting QFPs of 0.3 mm pitch of the leads that the reduction of the size of the openings cannot meet the situation. Moreover, the solder becomes short for the other existing components on the same circuit board if the mask is made thinner.
The necessary amount of the solder is different between the electronic components of a narrower pitch and the other existing components. As the difference is lately increasing more and more, a metallic mask of uniform thickness is not fit for every kind of printing. Although the half-processed mask, the thickness of which is partially changed as illustrated in FIG. 15, has been put to trial, the printing amount of the cream solder is actually irregular, and the cream solder blurs or bridges are generated, similar to the prior art.